In existing wireless technologies, signal repeating devices, such as repeaters or distributed antenna systems (DAS), are used to extend the coverage of an overall wireless system beyond the range of traditional base stations. For example, an overall cellular or wireless communication system may consist of a plurality of base transceiver stations (BTS) or base stations that communicate with each other and with user equipment, such as cellular phones, to provide a defined coverage area. In such coverage areas, there are often smaller geographical areas that have very low signal coverage, as provided by one or more of the base stations. For example, such areas of low signal coverage may be within buildings or in areas that are otherwise obstructed, such as by terrain features or man-made structures. Rather than simply implementing another costly and large base station to provide coverage in such low signal areas, repeaters and distributed antenna systems are often utilized.
Within buildings, a DAS system might incorporate one or more master units that receive downlink signals from one or more donor base stations and then distribute those signals via fiber optic or copper cable throughout the building. Waveguides or free-space laser links might be used as well. At designated points in the building, remote units coupled with the master unit(s) then amplify the downlink signals and connect them to radiating antennas. At those same points, uplink signals received from mobile users may be amplified, filtered, and sent back through the distribution system where they are summed together and transmitted back to the donor base station. At the remote units, the transmit (downlink) and receive (uplink) signals are usually combined onto a single antenna using a duplexer. The key function of the duplexer is to provide isolation between the transmitter or downlink signals and the receiver and uplink signals while connecting those devices and signal paths to a single antenna. Isolation between the transmitter and receiver is desirable to protect the sensitive receiver circuitry from the higher power transmit signals produced by the transmitter.
There are drawbacks, however, in using a duplexer to do such signal combining. First, duplexers are large devices and are expensive. Second, duplexers achieve their isolation by using fixed filters tuned to the specific frequencies that are sharing the antenna. For example, a duplexer includes two fixed tuned RF filters that are joined at one end for connection to a single antenna. One filter is tuned to the receive or uplink frequencies and the other is tuned to the transmit or downlink frequencies. Therefore, the remote units using such duplexers are frequency limited.
To be cost effective and flexible a DAS system remote unit needs to cover a wide range of frequencies, such as from about 400 MHz to about 5,000 MHz. The allocation of these frequencies into bands may change over time and are typically different in different countries. Fixed tuned duplexers provide little or no flexibility. For example, covering several bands may require several expensive and bulky duplexers and a switch matrix to select the proper duplexer for a given band. To build a low cost remote unit for such a system, a solution that does not require a duplexer is desired.
Embodiments of the present invention address the drawbacks in the prior art as discussed further below, and provide a significant advantage over a duplexer based system wherein the remote unit can only cover a single band, such as approximately an 824 MHz to 894 MHz band.